Supermarkets have been urged to put lids on their freezers in a new consumer report.

“The discounters Aldi and Lidl were the only two supermarkets to have closed doors on all freezers, thereby helping to conserve energy,” the body Consumer Focus said.

“We challenge all supermarkets to follow in the footsteps of their better rivals and help their customers – whatever their budget – to shop green.”

Meanwhile, Sainsbury’s has announced plans to cut the carbon footprint of its fridges by a third by 2030. It will stop using fridges cooled by so-called F-gases in favour of ones using carbon dioxide.

“Fridges are by far and away the biggest source of CO2 emissions in any supermarket through both the energy required to power them and the refrigerants themselves,” chief executive Justin King said. “If all supermarkets in the UK switched to this sort of refrigeration, the reduction in CO2 emissions would amount to around two million tonnes a year.”